add domain layer, config, and entry point

Domain: User, Session, EmailAddress, DTOs, repositories, services (PasswordHasher, TokenGenerator, Clock). Config: PHP-DI container definitions and Slim routes. Entry point: public/index.php with slim-bridge.

backend/app/Shared/ValueObject/EmailAddress.php

@@ -0,0 +1,54 @@
+<?php
+
+namespace App\Shared\ValueObject;
+
+use InvalidArgumentException;
+
+final readonly class EmailAddress
+{
+    private string $normalized;
+
+    private string $domain;
+
+    private const ERROR_MESSAGE = 'Invalid email address:';
+
+    public function __construct(string $email)
+    {
+
+        $trimmed = trim($email);
+
+        if ($trimmed === '' || ! str_contains($trimmed, '@')) {
+            throw new InvalidArgumentException(self::ERROR_MESSAGE." $email");
+        }
+
+        [$local, $domain] = explode('@', $trimmed, 2);
+        $this->domain = mb_strtolower($domain);
+        $normalized = $local.'@'.$this->domain;
+
+        if (filter_var($normalized, FILTER_VALIDATE_EMAIL) === false) {
+            throw new InvalidArgumentException(self::ERROR_MESSAGE." $email");
+        }
+
+        $this->normalized = $normalized;
+    }
+
+    public function value(): string
+    {
+        return $this->normalized;
+    }
+
+    public function equals(self $other): bool
+    {
+        return $this->normalized === $other->normalized;
+    }
+
+    public function getDomain(): string
+    {
+        return $this->domain;
+    }
+
+    public function __toString(): string
+    {
+        return $this->normalized;
+    }
+}